Nitrogen oxides are produced, inter alia, on combustion of fossil fuels in consequence of oxidation of the molecular nitrogen in the combustion air, as well as in consequence of oxidation of nitrogen chemically bound in the fuel, and they may also be found in flue gases or exhaust gases from nitric acid production and refuse disposal units. In today's increasingly environment-conscious society, it is of great importance to be able to remove efficiently nitrogen oxides from gas flows, especially exhaust gas flows from the combustion of various fuels, such as coal, gas, oil and petrol. In most cases, the commercially interesting solutions are based on the principle of reducing the nitrogen oxides to nitrogen by means of a suitable reducing agent, usually ammonia, a suitable catalyst being used to achieve an increased turnover and a lowered reaction temperature.
A suitable catalyst material in such methods is based on a combination of one or more transition metals and a carrier material. This catalyst material may be applied on a carrier body containing long and narrow channels forming a honeycomb structure, see for example U.S. patent specification No. 4,157,375. This catalyst type, however, involves the risk of clogging of the channels if the dust separation is not efficient enough, and also the size of these catalysts constitutes a problem, because they take up much space, which means that the installation and operation thereof are expensive, especially in regeneration.
Therefore, also particulate or powdery catalyst material is used. U.S. Pat. No. 4,518,710 discloses a powdery catalyst for the reduction of nitrogen oxide at about 250.degree.-450.degree. C., and which, according to the Examples, may be used packed in a vertical column.
WO Specification No. 86/05712 discloses catalyst particles in the order of 20-200 .mu.m for the reduction of nitrogen oxide at about 350.degree.-450.degree. C., the catalyst particles forming a suspension flow together with the treated gas when this is admitted to a reactor, whereupon the particles are separated therefrom after the reduction and recycled to the gas inflow position.
Similar catalyst materials are known from EP patent specification No. 268265, GB patent specification No. 1424486, U.S. Pat. No. 4,847,058, DE patent specification No. 3531809 and U.S. Pat. No. 4,742,037.
SE patent application No. 8700117-8 discloses a method for the reduction of nitrogen oxides in flue gases by means of ammonia and a powdery catalyst, the gas mixture being led through a suppression filter on whose surface a coating of catalyst material has accumulated, whereupon this material easily may be blown off for regeneration and thereafter reformed into a new catalytic layer.
This method is practically and economically advantageous in that the catalytic reaction can be carried out in already existing, inexpensive units, such as dust separators or filter traps. It is, however, desirable to find a catalyst fulfilling the requisite demands on pickup, layer build-up, and regeneration, at the same time as a high turnover can be achieved and a low temperature level be maintained.